Hydrodynamic and chemical conditions during onset of a red-tide assemblage in an estuarine upwelling ecosystem

Abstract

The hydrodynamics and nitrogen/silicon bio- geochemistry accompanying the development of a red- tide assemblage were examined in the RõÂa de Vigo (northwest Spain), a coastal embayment a ected by upwelling, during an in situ diel experiment in Septem- ber 1991. Despite a low N:Si molar ratio (0.5) of nutri- ents entering the surface layer, which was favourable for diatom growth, the diatom population began to decline. Limited N-nutrient input, arising from moderate coastal upwelling in a strati®ed water column, restricted net community production (NCP 630 mg C m)2 d)1). In addition, light-limitation of gross primary production (GPP 1525 mg C m)2 d)1) was observed. The rela- tively high f-ratio ( NCP:GPP) recorded (0.41, char- acteristic of intense upwelling conditions) would have been as low as 0.15 had not GPP been limited by light intensity. Temporal separation of carbohydrate synthe- sis during the photoperiod from protein synthesis in the dark could be inferred from the time-course of the C:N ratio of particulate organic matter. Severe light-limita- tion would lead to diatom collapse were the diatoms not able to meet all their energy requirements during the hours of darkness. Under the hydrodynamic, nutrient and light conditions of the experiment, an assemblage of red-tide-forming species began to develop, aided by their ability to migrate vertically and to synthesize carbohy- drates during the light in surface waters and protein during the dark at the 4 m-deep pycnocline. Thermal strati®cation, reduced turbulence, intense nutrient mineralization, and the limited nitrogen input through moderate upwelling were all favourable to the onset of a red-tide assemblage.